Types of Injection

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Making a class's dependencies explicit and requiring that they be injected into it is a good way of making a class more reusable, testable and decoupled from others.

There are several ways that the dependencies can be injected. Each injection point has advantages and disadvantages to consider, as well as different ways of working with them when using the service container.

Constructor Injection

The most common way to inject dependencies is via a class's constructor. To do this you need to add an argument to the constructor signature to accept the dependency:

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namespace AppBundle\Mail;

// ...
class NewsletterManager
{
    private $mailer;

    public function __construct(MailerInterface $mailer)
    {
        $this->mailer = $mailer;
    }

    // ...
}

You can specify what service you would like to inject into this in the service container configuration:

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services:
    # ...

    AppBundle\Mail\NewsletterManager:
        arguments: ['@mailer']

Tip

Type hinting the injected object means that you can be sure that a suitable dependency has been injected. By type-hinting, you'll get a clear error immediately if an unsuitable dependency is injected. By type hinting using an interface rather than a class you can make the choice of dependency more flexible. And assuming you only use methods defined in the interface, you can gain that flexibility and still safely use the object.

There are several advantages to using constructor injection:

  • If the dependency is a requirement and the class cannot work without it then injecting it via the constructor ensures it is present when the class is used as the class cannot be constructed without it.
  • The constructor is only ever called once when the object is created, so you can be sure that the dependency will not change during the object's lifetime.

These advantages do mean that constructor injection is not suitable for working with optional dependencies. It is also more difficult to use in combination with class hierarchies: if a class uses constructor injection then extending it and overriding the constructor becomes problematic.

Setter Injection

Another possible injection point into a class is by adding a setter method that accepts the dependency:

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// ...
class NewsletterManager
{
    private $mailer;

    public function setMailer(MailerInterface $mailer)
    {
        $this->mailer = $mailer;
    }

    // ...
}
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services:
    # ...

    app.newsletter_manager:
        class: AppBundle\Mail\NewsletterManager
        calls:
            - [setMailer, ['@mailer']]

This time the advantages are:

  • Setter injection works well with optional dependencies. If you do not need the dependency, then just do not call the setter.
  • You can call the setter multiple times. This is particularly useful if the method adds the dependency to a collection. You can then have a variable number of dependencies.

The disadvantages of setter injection are:

  • The setter can be called more than just at the time of construction so you cannot be sure the dependency is not replaced during the lifetime of the object (except by explicitly writing the setter method to check if it has already been called).
  • You cannot be sure the setter will be called and so you need to add checks that any required dependencies are injected.

Property Injection

Another possibility is just setting public fields of the class directly:

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// ...
class NewsletterManager
{
    public $mailer;

    // ...
}
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services:
    # ...

    app.newsletter_manager:
        class: AppBundle\Mail\NewsletterManager
        properties:
            mailer: '@mailer'

There are mainly only disadvantages to using property injection, it is similar to setter injection but with these additional important problems:

  • You cannot control when the dependency is set at all, it can be changed at any point in the object's lifetime.
  • You cannot use type hinting so you cannot be sure what dependency is injected except by writing into the class code to explicitly test the class instance before using it.

But, it is useful to know that this can be done with the service container, especially if you are working with code that is out of your control, such as in a third party library, which uses public properties for its dependencies.

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.
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